Among many ways to heat water, both a gas heater and an electric heater are usually used. The electric heater has already been more and more popular recently since it can be installed conveniently in many places. According to functions of the electric heaters, electric heaters can be divided into two modes, one is a storage mode and the other is an instant heating mode. The disadvantages of a storage heater includes its large size, high price, slow heating. Besides, the hot water stored in a storage heater needs to be kept warm all day long, which causes electric energy to be wasted. In use, the temperature of the hot water will decrease abruptly, because the hot water was not drained out by its heating sequence and was mixed with the entering cool water. For an instant heating heater, it is difficult to adjust a suitable temperature for usage. Although a multi-stage temperature adjuster can be used in an instant heating heater, it is also inconvenient during changes of whether. If the temperature adjuster is installed, the power of the instant heating heater cannot be turned on by a faucet to heat the cool water. In addition, the instant heater needs a great quantity of electric current which causes waste of electric energy.